In recent years, there has been increasing interest in the use of heat treatment (hyperthermia) for controlling or preventing the growth of malignant cells. While hyperthermia has some demonstrated utility as a stand-alone modality, it shows most promise when used as an adjunct therapy in combination with other treatments such as ionizing radiation or chemotherapy. In both patient and animal testing, it has been found that heating tumors to temperatures in the range of 41.degree.-45.degree. C. can reduce at least 50% of tumor masses in at least 50% of the subjects treated. Although the reason for this is not yet well understood, it appears that malignant cells are inherently more sensitive to heat than normal cells, and furthermore that the thermal sensitivity of cells in poorly perfused regions (such as solid tumors) is increased. It is also noted that where the tumor is poorly perfused, it is easier to induce local hyperthermia.
The use of ultrasound to induce hyperthermia is well-known, and the basic method of ultrasonic treatment is generally as follows. An ultrasonic transducer is acoustically coupled to the subject's skin, taking care to avoid an air-tissue interface which would cause reflection. The transducer is electrically driven at a frequency of several hundred kilohertz which causes pressure waves to propagate into the tissue. The tissue becomes heated by the dissipation of the ultrasonic pressure waves. The intensity distribution associated with the waves is characterized by a prominant central (on-axis) peak. The heat generated in the tissue is a function of the time averaged intensity distribution of the pressure waves and further depends on such factors as the perfusion of the tissue.
As is well known in the art, there are other methods of inducing local hyperthermia, namely microwave radiation, and induced radiofrequency current heating (either directly applied or electromagnetically induced), but ultrasonic techniques have certain advantages. More particularly, the ultrasonic transducers may be constructed to permit scanning and to provide well-collimated beams which can be easily focused and manipulated. Additionally, ultrasound is not associated with biohazard and does not interfere with temperature measurements which are necessary to control the hyperthermia treatment. Moreover, ultrasound does not preferentially heat fatty tissue, although it is noted that bone has an appreciably higher rate of absorption.
However, to date, ultrasonic treatment has been largely restricted to depths of at most a few centimeters, and attempts to deliver usable amounts of power to greater depths (for example, 15 cm) have tended to result in unacceptable overheating of the surface and near surface tissue. Moreover, systems for treating the shallow regions have been plagued by phase interference effects. Therefore, in spite of the great promise shown by the use of ultrasonically induced hyperthermia for tumor treatment, the range of applications has been limited so that the full potential of the treatment has not been realized.